The present invention is concerned, in general terms, with the technical field of tower cranes. This invention relates more particularly to the latticework jibs of tower cranes, and, even more specifically, its subject is a device for demountable assembly of the jib elements of a tower crane. The invention applies more especially to the assembly of the component elements of a crane jib not comprising a projecting part, known as a masthead or jib carrier, projecting above the upper chord of the jib and counterjib and associated with ties.
In a generally known way, a tower crane jib, along which the jib trolley is conventionally displaced, consists of a succession of jib elements which are aligned and assembled with one another so as to form a jib having the desired length. Each jib element is a structure of the latticework girder type, of triangular, rectangular or trapezoidal cross section, which comprises chords which in pairs define plane faces. In each of these plane faces, the two chords are connected to one another by means of elongate pieces of a bar type which together form what is called a “triangulation”. This type of structure is also used for the counterjibs of tower cranes which support a counterweight balancing the jib and, where appropriate, the load raised by the crane.
Inasmuch as a foldable crane jib is not concerned, the component elements of the jib of the crane must be capable of being separated from one another for the transport of the crane, and these jib elements have to be assembled with one another, at the place of use of the crane, for the purpose of reforming a crane jib which can be used. The jib elements must therefore, as far as possible, be capable of being assembled with one another easily and also of being conveniently separable from one another.
Moreover, the connection to be made between such jib elements must be adapted to the forces to which these jib elements are subjected, particularly when the crane is operating. If the elements of the “cantilevered” part of the jib of the crane are considered more particularly, the upper chords of these elements are subjected to tensile forces during operation or even at rest, whilst the lower chords of the same jib elements are subject to compressive forces.
The case of the aligned jib placed on the ground on two supports must also be taken into consideration; this is an assembly configuration in which the upper chords of the jib elements are subjected to compressive forces, while the lower chords undergo tensile forces.
The situation where the sling breaks must also be taken into consideration; this is a test case corresponding to the breakage of the slings securing a raised load or to the breakage of the lifting cable, thus giving rise to initially vertical forces which react on the jib elements and on their connections. More particularly, in the event of a sling breakage, the upper chords of the jib elements are subject to compressive forces, while the lower chords of these jib elements undergo tensile forces.